Add GET_LOCK to prevent concurrent migrations of the same table

[jakubpliszka]

Description

This PR prevents two gh-ost processes from migrating the same table concurrently on the same MySQL server.

The existing table existence check runs only on fresh start, and is defeated by the dummy-drop-and-retry recovery flow when the previous gh-ost is still alive.

initiateApplier now takes GET_LOCK('gh-ost::<db>.<table>', 0) on a pinned connection. The lock survives table cleanup, auto-releases on TCP close so retries work, and a keepalive triggers PanicAbort if the connection dies.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

Adds a MySQL user-level GET_LOCK taken on a dedicated pinned connection at the start of initiateApplier() to prevent two gh-ost processes from concurrently migrating the same table on the same MySQL server. A keepalive goroutine pings the pinned connection and triggers PanicAbort if the connection dies; the lock is released in Applier.Teardown().

Changes:

• New AcquireMigrationLock/releaseMigrationLock plus a keepalive goroutine in Applier, called from Migrator.initiateApplier() and released via Applier.Teardown().
• Lock name gh-ost::<db>.<table> (SHA1-hashed if it exceeds MySQL's 64-char limit) computed by buildMigrationLockName.
• New testcontainer-backed tests for acquire-when-free and acquire-when-held, a unit test for buildMigrationLockName, and added defer migrator.applier.Teardown() in migrator copier tests.

Show a summary per file

File	Description
go/logic/applier.go	Adds migration-wide GET_LOCK infrastructure (pinned conn, keepalive, release) and wires release into Teardown.
go/logic/migrator.go	Acquires the migration lock immediately after InitDBConnections in initiateApplier.
go/logic/applier_test.go	Adds integration tests for lock acquisition success/failure and a unit test for the lock-name builder.
go/logic/migrator_test.go	Ensures applier is torn down after initiateApplier() in copier tests so the new lock connection is released.

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• Files reviewed: 4/4 changed files
• Comments generated: 0